Infrared-radiation thermometers used in professional applications as in the case of infrared fever thermometers at hospitals require periodic functional testing to permit timely detection of undesirable contamination or possible calibration errors and correspondingly incorrect measuring results. For that purpose, hospitals typically install a black body radiator of a known temperature to serve as the radiation reference point. Such radiators, however, are relatively expensive and their operation is awkward insofar as it is necessary for each routinely required functional test to transport the infrared fever thermometers back and forth between the station concerned and the radiation reference point.
WO 00/54012 describes an infrared sensor consisting of a sensor enclosure with an infrared-transmissive radiation incidence window, a detector element housed in the sensor enclosure serving to convert the incident infrared radiation into electrical measuring signals, and associated measuring electronics for capturing and evaluating the measuring signals. Outside the beam path, the sensor enclosure also incorporates an activable infrared radiator so positioned that at least part of the energy emitted by it passes through the radiation incidence window, heating up any contaminants that might be present on it. The detector element captures the temperature gradient and converts it into a calibration signal, which then serves as a measure of the contamination of the window and is used in the automatic compensation for the contaminating effect in a subsequent temperature measurement.